Cellular or mobile networks are radio networks which may be distributed over a large geographical area. This geographical area is divided into “cells.” Each cell is generally served by at least one transceiver known as a “base station,” which is often fixed in location, and may comprise an evolved node B (eNB) on a Long Term Evolution (LTE) system. Together, the base stations may comprise a wireless wide area network (“WWAN”). The WWAN can also be communicatively coupled with a public or private network, which may include that particular aggregation of networks commonly known as the Internet.
The cellular network may comprise both “macrocells” and “small cells.” A macrocell provides radio coverage served by a high power cellular base station, which typically has power outputs of tens of watts, and may be mounted on ground-based masts, rooftops, and other existing structures, at a height that provides a clear view over surrounding buildings and/or terrain. Small cells, on the other hand, are generally low-powered radio access nodes, encompassing, for instance, femtocells, picocells, and microcells. Whereas a macrocell may have a range of a few kilometers or more, small cells generally have ranges of less than a couple kilometers (e.g., In a rural setting), and frequently within the range of a couple hundred meters or less (e.g., 10 meters within an urban setting). Mobile operators often use small cells to extend their service coverage and/or increase network capacity, for example, by offloading traffic from macrocells to small cells during peak traffic times.
User equipment, which may be mobile and moving, is configured to establish connections with the base stations of the macrocells and small cells which form the cellular network. As used herein, the term “user equipment” (UE) may refer to any type of device, including, without limitation, a mobile station, such as a mobile communication device (e.g., smart phone or other wireless phone), tablet computer, and/or laptop computer, as well as a desktop computer. The connections formed between UEs and base stations may be established, for example, using code division multiple access (“CDMA”), Global System for Mobile Communications (“GSM”), Universal Mobile Telecommunication System (“UMTS”), LTE, or the like. Through these connections with the base stations, the UEs are able to establish voice and/or data communications with each other and other transceivers or receivers within the network or within other connected networks, including the Internet.
Small cells, such as femtocells, picocells, and microcells, may operate within the coverage area of large macrocells to form a heterogeneous network. Such a network can provide uniform broadband services across the entire coverage area of the macrocells. However, when the macrocell and small cells operate on the same radio carrier, co-channel interference between cells may cause a degradation in the performance of the cellular network. While operators may avoid such interference by allocating separate radio carriers to the macrocells and small cells, this can impact carrier utilization efficiency, and operators may be too limited in the amount of available spectrum to afford such an allocation.
A common technique to mitigate co-channel interference between UEs being served by a macrocell (“macrocell UE”) and UEs being served by a small cell (“small-cell UE”) is to allocate some of a carrier's sub-carriers for serving UEs that are nearer to the center of the macrocell and other sub-carriers for serving UEs that are nearer to the edge of the macrocell. The small-cell UEs are served using the sub-carriers allocated for the macrocell UEs nearer to the center of the macrocell. In some deployments, a small cell may also use the sub-carriers allocated for macrocell edge UEs, but the transmission power of the small cell UEs is restricted. Thus, the macrocell UEs nearer to the edge of the macrocell do not interfere with the small-cell UEs, and the macrocell UEs nearer to the center of the macrocell are separated by distance from the small-cell UEs. When a macrocell UE moves close to the edge of the macrocell and close to the small cell, measurement reports from the macrocell UE can trigger the macrocell to restrict the UE's resource assignments to those sub-carriers allocated for macrocell UEs nearer to the edge of the macrocell.
In a similar manner, operators may wish to deploy multiple carriers within the coverage areas of macrocells and small cells and dynamically control the use of these carriers in order to increase spectrum efficiency. Interference can be controlled by allocating a carrier or carriers for macrocell UEs nearer to the center of the macrocell and small-cell UEs, and allocating a different carrier for macrocell UEs nearer to the edge of the macrocell's coverage area and nearer to the small cell. When the macrocell UE moves close to the macrocell edge and close to the small cell, it can be handed over to the other macrocell carrier or to the small cell in order to avoid interference with the small cell.
However, FIG. 1 illustrates a scenario in which a UE being served by a macrocell may interfere with the operation of a small cell without normally triggering a handover. As shown, small cell 130 operates within the coverage area 122 of macrocell 120 and has unbalanced or asymmetrical uplink and downlink coverage. Specifically, uplink range 132 represents the area in which the small cell is able to receive an uplink signal from UE 110, and has greater coverage than downlink range 134, which represents the area in which the UE 110 is able to receive a downlink signal from small cell 130. While small cell 130 is shown as operating entirely within the range 122 of macrocell 120, it should be understood that the following description of an interference scenario applies to any instance in which at least a portion of the uplink range 132 of small cell 130 is within the coverage area 122 of macrocell 120.
Macrocell 120 and small cell 130 operate on one or more common radio carriers, such that UEs utilizing the macrocell, such as UE 110, and UEs utilizing the small cell 130 may experience co-channel interference. For example, in the scenario illustrated in FIG. 1, UE 110 is initially being served by macrocell 120, and is operating on at least one radio carrier frequency that small cell 130 may utilize for the UEs being served by small cell 130. UE 110 may have been assigned the common carrier either as a primary cell (PCell) or secondary cell (SCell). UE 110 is within the uplink coverage area of small cell 130, but is not within the downlink coverage area of small cell 130. In other words, the small cell 130 can receive uplink signals from the UE 110, but the UE 110 is unable to receive downlink signals from the small cell 130. Consequently, UE 110's uplink signal is interfering with small cell 130, but UE 110 is unable to detect small cell 130.
If the downlink and uplink signals were balanced, the downlink signal strength at UE 110 could trigger a measurement event of the UE 110. In the case without uplink-downlink imbalance, the UE 110 would measure the downlink signal strength of small cell 130. The UE 110 would then send the measurement information to macrocell 120. This transmission of measurement information would normally inform macrocell 120 about the potential interference at small cell 120 caused by UE 110. Based on the measurement information, macrocell 120 would determine what corrective action, if any, should be taken. For example, macrocell 120 may initiate a handover procedure to hand over UE 110 to be served by small cell 130, or to continue to be served by macrocell 120 but on a different radio carrier.
However, in the scenario illustrated in FIG. 1, the downlink signal strength from the small cell 130 is not strong enough at the UE 110 to trigger a measurement event of the UE 110. Thus, the macrocell 120 is unaware that the UE 110 is causing interference for small cell 130, and therefore, unable to mitigate the interference through corrective action, such as a handover. The small cell 130 could report the uplink interference to macrocell 120, for example, over a terrestrial connection between the two base stations, such as an X2 interface of the LTE standard. However, even if macrocell 120 is informed of the interference, it would be unable to take corrective action since the macrocell may be servicing multiple UEs, and the identity of the particular UE causing the interference is unknown to both macrocell 120 and small cell 130.
Accordingly, there is a need for mechanisms to control the use of radio carriers in the operation of a cellular network. The LTE specifications include several mechanisms to assist with controlling interference. UEs are capable of providing a serving base station with measurement reports of the downlink signal strength of neighbor cells. These have been commonly used in mobile systems to assist with handovers to other cells. For interference management, the measurement reporting capability has been expanded to allow a base station to determine whether the UE is operating within the central region of the cell or whether it is operating in the edge region of a cell and is near another cell. New triggers for downlink signal strength measurement reporting have been defined to help the macrocell determine when a UE is crossing the boundary between the central region and the cell edge region. However, these triggers are dependent on the UE detecting and measuring the downlink signal strength of a neighbor base station, and a common technique for controlling downlink interference from a small cell is to reduce the small cell's downlink transmission power, which is one illustration of how an imbalance can occur between the uplink and downlink coverage areas of the small cell, as illustrated in FIG. 1.
Another mechanism provided in the LTE specifications to assist with controlling interference is the uplink (UL) High Interference Indication (HII). This parameter may be used by a base station to notify other base stations about sub-carrier allocations being allocated for resource assignments for UEs near the edge of a cell. Base stations receiving this parameter should avoid assigning these resources to their served UEs, or should only assign these resources to UEs transmitting with lower power. Another LTE mechanism for interference control is the Uplink Interference Overload Indication (OI) parameter, which may be exchanged between base stations and provides uplink interference information about one of the sending base station's cells where the particular sub-carriers with uplink interference are identified for a particular carrier, the uplink interference measurements have been averaged over a time period, and the uplink interference is likely being caused by one of the receiving base station's cells. In some implementations, OI may be used in conjunction with HII to notify a base station when the interference levels dictated by the base station's HII parameters are not being met, such that the uplink interference levels of a cell of another base station have exceeded a threshold. An illustrative case would be where a small cell is operating on the same frequency resources as cell-centric macrocell UEs, and one or more macrocell UEs are causing excessive uplink interference to the small cell. The small cell can report this interference to the macrocell using the OI parameters, and the macrocell might change the boundary between its cell-centric and its cell-edge regions. Another illustrative example is the case where a small cell makes lower power UL transmission assignments to UEs on the frequency resources that a macrocell has allocated for its cell-centric UEs, and these small cell UEs are causing excessive uplink interference for the macrocell. The macrocell base station may send OI parameters to the small cell, and the small cell may reduce the transmission power of UEs transmitting on these frequency resources.
These current mechanisms for base stations to exchange uplink interference information, comprised of identifying and reporting sub-carriers with excessive interference over a time period, are not effective for reporting and resolving uplink interference in the scenario illustrated in FIG. 1. Specifically, in the illustrated scenario, there is a need to identify a macrocell UE that is causing interference to a small cell. A macrocell UE may be assigned different uplink sub-carriers for each transmission, depending on the radio conditions at the time of assignment. Thus, identifying the sub-carriers on which the interference occurs does not help to identify the interfering UE. In addition, the small cell uplink interference caused by a macrocell UE may not meet the criteria for triggering an OI report, but may still cause significant interference to the small cell. What is needed is a novel uplink interference indicator that may be exchanged between base stations.